Various solutions exist to regulate an output voltage of an unregulated power source, such as a charge pump, to provide a stable and steady output voltage. Some solutions are based on systems that compare the charge pump output voltage with a reference voltage and then generate an output signal that stops the clock feeding the charge pump when the desired voltage has been reached. However, due to the unavoidable propagation delay of the control loop in stopping the clock and the periodic fluctuation of the output voltage amplitude due to the pumping action, voltage ripples will be observed at the regulated voltage output. Although there are ways to minimize the ripples, they cannot be eliminated completely. As a result, these ripples can present a problem for those applications requiring a very precise voltage source.
Alternatively, a serial regulation scheme, in which a pass device is connected between the unregulated charge pump output and the regulated voltage output, provides a smoother voltage output. The pass device, being controlled by a feedback loop furnished with an error amplifier, sinks an appropriate amount of current in response to the output of the error amplifier, thereby maintaining a stable and precise voltage output. However, current consumption for such devices is high at high voltage output because the error amplifier must be supplied by the charge pump output to properly drive the gate of the pass device. Another source of current consumption is the bias current going through the pass device.
Therefore, it would be desirable to have a voltage regulation system that provides a stable and precise voltage output without the drawback of high current consumption.